1. Field of the Invention
This invention relates to a limestone (lime)-gypsum flue gas desulfurization process. More particularly it relates to a wet type flue gas desulfurization process by removing sulfur oxides (SO.sub.x) contained in flue gases, with limestone (CaCO.sub.3) or caustic lime (Ca(OH).sub.2) as an absorber, and at the same time recovering gypsum (CaSO.sub.4.2H.sub.2 O) as byproduct, and apparatus employed therefor.
According to the above desulfurization process, sulfur oxides (SO.sub.x) (usually SO.sub.2 or SO.sub.3) contained in flue gases are converted into gypsum which is chemically stable and utilizable, with CaCO.sub.3, CaO or Ca(OH).sub.2 as raw material, which are abundantly present in nature and cheaply available, and the flue gases are made harmless.
2. Description of the Prior Art
Conventional flue gas-desulfurization processes comprise a cooling step, and absorption step, a concentration step for slurry of gypsum formed in the absorption step, and an exhaust gas-heating step. Exhaust gas is led to the cooling step wherein it is partly desulfurized and at the same time cooled and dedusted. It is then led into an absorption tower wherein it is further desulfurized. The resulting gas is sent to the exhaust gas-heating step, heated therein and delivered. On the other hand, a slurry containing limestone and lime is led to the absorption step wherein it absorbs SO.sub.x and byproduces CaSO.sub.3 and Ca(HSO.sub.3). A portion of the resulting slurry containing unreacted CaCO.sub.3 and Ca(OH).sub.2 is sent to the cooling step wherein the amounts of unreacted CaCO.sub.3, etc. are reduced. Further, portions of the byproducts CaSO.sub.3 and Ca(OSO.sub.3).sub.2 are oxidized in the absorption and cooling steps to form gypsum (CaSO.sub.4). The slurry used in the cooling step is sent to the concentration step wherein it is separated into water and concentrated solids, the water being reused, and the concentrated solids (CaSO.sub.3, Ca(HSO.sub.3).sub.2, and CaSO.sub.4) being stored in a pond close to the concentration step. In this case, water resulting from precipitation of the solids in the pond may be reused.
The above conventional processes, however, raise such problems that since the byproducts are a mixture of CaSO.sub.3, Ca(HSO.sub.3).sub.2 and gypsum, and partly contain unreacted CaCO.sub.3, they cannot be effectively utilized even if recovered, and also since fine particles of CaSO.sub.3 and Ca(HSO.sub.3).sub.2 having a particle size of several .mu.m are present in a large amount in the slurry in the concentration step, the settling property is so poor that it is necessary to make the concentration apparatus larger. Further, in the case where a slurry of Ca compounds is used as an absorbing agent, since the byproducts (CaSO.sub.3 and CaSO.sub.4) cause scaling, it is necessary to take a countermeasure therefor.
From such a standpoint, a process has been desired which is capable of recovering gypsum as byproduct and also recovering the gypsum according to various uses, and also which causes no scaling, etc., is highly reliable and is very efficient employing as minimum equipments as possible, from the viewpoint of a process efficiency.